1. Field of the Invention
The invention relates to apparatus and methods for milling holes in wellbore casings of the type used for hydrocarbon production, and especially those wellbores in which coiled tubing is used to initially lower a milling device and subsequently lower a jet drilling hose to the bottom of the wellbore. In one of its aspects, the invention relates to a method and an apparatus for transferring a known amount of weight to a bit to mill a hole in a wellbore casing. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing in a relatively quick and cost effective manner. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing that is deviated. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing at greater depths than heretofore possible. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing wherein the skill of the operator in controlling the operating tools is lessened. In another of its aspects, the invention relates to a method and an apparatus for milling a hole in a wellbore casing wherein the tools are less expensive to build and operate. In another of its aspects, the invention relates to a method and an apparatus for milling multiple holes in a wellbore casing without removing the cutting tools from the wellbore.
2. Description of Related Art
Hydrocarbon wellbore casings often have lateral holes milled in them using a small diameter motor-driven “knuckle” joint drive assembly with a bit on the leading end. The motor used is often a fluid-driven motor known as a mud motor, lowered on the end of standard coiled tubing. Once the holes are milled the milling equipment is removed, and the coiled tubing is subsequently used to lower a jet drilling assembly down to where it can be pushed out through the milled holes to drill into the surrounding well formation.
Using a motor-driven knuckle joint drive for the milling operation entails several problems for the operator. The lowered knuckle joint drive assembly is poorly stabilized during the cutting operation, requiring additional time to cut a hole in the casing. Lowering the knuckle drive assembly requires significant skill on the part of the operator, particularly when standard size coiled tubing is used, since the operator has virtually no “feel” over the milling operations and must depend on surface gauges hundreds or thousands of feet above to determine how to control the milling operation. Some of the available torque from the motor is expended on frictional drag resulting from the joint assembly rubbing against the inside of the deflector shoe, or resulting from the coiled tubing rubbing on the inside wall of the production tubing or “work string”, making it even more difficult for the operator to know how much torque is available for the milling operation. Wellbores with increased deviation angle reduce the amount of weight that can be transferred to the bit via the knuckle drive for the milling operation. Small diameter knuckle joint assemblies cannot be used in high angle or horizontal wells; they make it difficult to know how much torque is really reaching the milling bit; and they make it difficult to know when the bit has completed milling a hole in the casing.
Alternatives to knuckle drive assemblies exist, but they also have drawbacks. One alternative is jointed pipe with a milling bit on the end, used in conjunction with a whipstock to drill a slot in the side of a wellbore casing. But conventional jointed pipe is time-consuming to put together and take apart on the surface, which is of particularly concern with wells drilled for hydrocarbon production because it results in high operating expense due to labor, rig rental, etc.
Another alternative uses coiled tubing to drive a jet nozzle using abrasive cutting fluids to cut a hole in the casing. But abrasive cutting fluid rapidly deteriorates and damages the pumping and metering equipment at the surface.